RRI necessary or alternative for variable

Hi experts

We have a BEx Query:

One filter variable describes product group.

Each product group has several products assigned.

In the same filter area a variable is defined on product.

Now the user should only select those products which are assigned to a product group.

The product group value is selected initially, thereafter the user should be enabled to select the valid products if necessary.

Is there only the possibility via RRI?

Example:

Product group   product

A1                      machine1

A1                      machine2

A1                      machine3

Thanks

T.